According to the biography of Picasso available via eNotes, Picasso started out his career as a student of his father, who was an art teacher and museum curator. He was originally trained in realistic-looking art: Picasso's father had him
copy the works of the masters with meticulous fidelity and pay close attention to the traditional laws of proportion and harmony of color.
This endeavor, which is reflected in his early work, features a more representational style, meaning that the art accurately represents reality. An example of one of Picasso's great works that is representational is "The Tragedy" from his famous Blue Period. It depicts a destitute trio of folks who seem to be mourning or suffering. They appear realistic, as do other paintings from Picasso's early years, Blue Period, and Rose Period.
Picasso is, of course, most famous for his abstract work, and there are many extraordinarily famous examples. One of the most well known is Guernica, a mural sized painting depicting the horrors of war. This work is clearly not representational--the figures are fragmented, with floating heads and members. The angles are not true to any type of discernible perspective. Geometric shapes intersect two-dimensional faces. This painting is absolutely abstract. Yet, it has an incredibly affecting impact that represents an abstract concept (the horrors of war) rather than copying the appearence of a physical scene. This type of unique work is Picassso's claim to fame.